The transmission of high-speed data via cable television systems involves the bi-directional transfer of Internet Protocol (IP) traffic over a cable system, typically an all-coaxial or hybrid fiber-coax (HFC) cable network. More particularly, and with reference to FIG. 1, the transmission path over such a cable system is realized at the head end by a cable-modem-termination system (CMTS) and at each customer location by a cable modem (CM). At the head end (or hub), the interface to the data-over-cable system is known as the cable-modem-termination system network-side interface (CMTS-NSI). At the customer locations, the interface is known as the cable-modem to customer-premises-equipment interface (CMCI). The system functions to transparently transfer IP traffic between these interfaces, including, but not limited to, datagrams, Dynamic Host Configuration Protocol (DHCP), Internet Control Message Protocol (ICMP), and IP group addressing (broadcast and multicast).
To facilitate the early definition, design, development, and deployment of data-over-cable systems on a uniform, consistent, open, non-proprietary, multi-vendor interoperable basis, a Multimedia Cable Network System (MCNS) consortium has promulgated Data-Over-Cable Service Interface Specifications (DOCSIS). One aspect of DOCSIS that is of particular importance to the present invention is the radio-frequency interface specifications to which vendors must conform in developing and field testing hardware. More particularly, DOCSIS provides detailed specifications for the electrical characteristics and communication protocols that define an interoperable CM and CMTS system.
Communication protocols are necessary because the CM and CMTS can operate both as forwarding agents and as end systems (hosts). Because these modes differ, the protocol stacks required for these modes must accommodate these differences. In addition, certain management functions ride on the IP packets. Shown in FIG. 2 is a protocol stack on the RF interface. The DOCSIS protocols also show, as depicted in FIG. 3, the CM and CMTS stacks for data forwarding through the CM and CMTS. Under the DOCSIS requirements, forwarding of IP traffic must be supported by the CM and CMTS.
Consequently, the CM must include configurable hardware and related software that meets DOCSIS requirements for establishing and maintaining connectivity to the cable system network.